Method of and apparatus for fabricating a tool to form an asymmetrical constant cross section bore in the propellant in a solid rocket motor

ABSTRACT

An elongated flexible bag is fitted inside templates that have aligned openings therein corresponding in shape to that desired for a solid rocket motor propellant bore. With the bottom sealed, the bag is filled with a multiplicity of solid pieces each having a cross section substantially less than that of the bag. When filled, suction is applied to the bag by a vacuum needle to exhaust air therefrom until the bag is very tight on the solid pieces contained therein. The vacuum needle is then withdrawn, the hole made thereby is sealed, and the templates are removed from the bag. After use in loading a solid rocket motor with propellant, the bag is cut open, the solid pieces are removed, and the bag is peeled out of the bore in the propellant.

This is a divisional of co-pending application Ser. No. 851,186 filed onApr. 14, 1986 pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a tool and a method for the fabricationthereof for forming bores of unusual or asymmetrical cross section inthe propellant in solid rocket motors.

2. Description of the Prior Art

Solid propellant rocket motors utilize a thin-wall cylindrical rocketmotor case that is closed at the forward end and has a nozzle at theother end for the emission of exhaust gases from the case. A case-bondedsolid propellant may be cast into the rocket motor case after a core offixed and desired configuration, generally made of metal, has beenplaced in the case with the propellant filling the space between thecore and the case, after which the propellant grain is cured. The coreis then removed leaving a cavity or bore having a constant or unchangingcross section the shape of which corresponds to the constant crosssection of the core, thus duplicating the exterior contour of the core.

The core may possess a plurality of star points that are arrangedsymmetrically with relation to the longitudinal axis of the core, thenumber of star points varying according to the individual requirement ofthe particular solid propellant rocket motor that is involved.

Solid propellant rocket motor cores may also possess unusual orasymmetric geometries such as, but not limited to, a "hook" asdisclosed, for example, in U.S. Pat. No. 3,529,550 to Ralph W. Snowden,issued on Sept. 22, 1970 and assigned to the assignee of the presentinvention. Such cores enable many variations in the characteristics ofthe solid propellant grain such as are needed to meet specificrequirements of a solid propellant rocket motor and such as occur whenthe number of star points of a symmetrical core is varied. Cores of suchunusual configuration have advantages including high loading density andreduction of strains in the solid propellant grain that are encounteredby the solid propellant rocket motor under temperature changes thatoccur during its operational requirements.

The fabrication, however, of metal cores in accordance with the practiceof the prior art to form unusual or asymmetrical constant cross sectionbores for use in solid rocket motors requires complex, and hence,expensive machining. The "hook" configuration is complicated and, ingeneral, can be made only with the use of three-dimensional numericalcontrol (NC) controlled machining capability.

Thus, there is a need and a demand for improvement in the tools and themethod of the fabrication thereof for forming such tools quickly withlittle requirement for special machining capability. The presentinvention was devised to fill the technological gap that has existed inthe art in this respect.

SUMMARY OF THE INVENTION

An object of the invention as to provide an improved method of andapparatus for fabricating a tool or core to form an asymmetrical,constant cross section bore in the propellant in a solid rocket motor.

In accomplishing this and other objectives of the invention, anelongated flexible bag is fitted inside templates that have alignedopenings therein that are shaped to correspond to the cross sectiondesired for the bore in the propellant of a solid propellant rocketmotor. With one end sealed, the bottom end, the bag is filled with amultiplicity of individual solid pieces of matter each of which issubstantially smaller in cross section than that of the bag. As aresult, when filled with such pieces, the bag is made to conform incross sectional shape to that of the shaped openings in the templates.

In one embodiment the individual pieces may comprise small rods of roundcross section. In another embodiment the individual members may comprisesmall rods of hexagonal cross section. In still another embodiment, theindividual members may comprise particles of granular material such assand.

When the bag has been filled with the multiplicity of pieces, the topend of the bag is sealed and air is exhausted or pulled from the bag bymeans of a vacuum needle until the bag is very hard on the bundle ofrods or mass of granular material therein. The vacuum needle is thenwithdrawn, the hole made in the bag by the needle is sealed, and thetemplates are removed from the bag.

The resulting rigid structure is then ready for use as a tool or corefor loading a solid rocket motor with propellant. After such loading andcuring of the propellant, which loading may be effected in a mannerknown in the prior art as described hereinbefore, the bag may be cutopen, the rods or other material contained in the bag may be extracted,and the plastic of which the bag is made may be peeled out of the borein the propellant.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages, and specific objects attained by its use,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

With this description of the invention, a detailed description followswith reference being made to the accompanying drawings which form partof the specification and in which like parts are designated by the samereference numerals, of which drawings:

FIG. 1 is a transverse sectional view of the propellant in a solidrocket motor illustrating the configuration of a typical asymmetricalconstant cross section bore therein, specifically a hook-shaped bore;

FIG. 2 is a fragmentary perspective view of a tool, at an intermediatestage in the fabrication thereof, that is made in accordance with theinvention and in which a multiplicity of individual pieces employed inthe fabrication thereof comprise small rods;

FIG. 3 is a fragmented perspective view of the completed tool accordingto the invention;

FIGS. 4 and 5 are views showing alternative cross sections that may beused for the rods employed in the embodiment of FIGS. 2 and 3 in thepractice of the invention; and

FIG. 6 is a fragmentary sectional view of a tool, at an intermediatestage in the fabrication thereof, according to another embodiment of theinvention, in which the multiplicity of individual pieces employedcomprise grains of sand.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, the numeral 10 in FIG. 1 designates a solidpropellant grain of a rocket motor in which an asymmetrical, constantcross section bore or cavity 12 having a hook shape is formed. The bore12, as shown, includes a bulbous end 14, a curved hook formation 16 anda shank 18 that joins the end 14 and the curved hook formation 16 into aunitary configuration having a hook shape.

A tool 20 according to the invention that may advantageously be employedin the formation of the hook-shaped bore 12 in the propellant 10 of FIG.1 is shown in FIG. 3. The tool 20 comprises an elongated plastic bag 22of suitable size which may be made of Teflon or other suitable material.Bag 22 is filled with a multiplicity of small elongated rods 24 that maybe made of any suitable solid material such as wood, plastic, metal,etc., and that may have a round or hexagonal cross section, as shown inFIGS. 4 and 5, respectively. Rods 24 may have a length correspondingsubstantially to that of bag 22. After being filled with rods 24 whileheld constrained in suitably shaped openings in templates 26 and 28 asshown in FIG. 2, bag 22 is sealed and the air therein is exhausted bymeans of a vacuum needle 30 to which suction at a suitable vacuumpressure is applied from a source (not shown). Such suction is appliedby means of needle 30 until the bag 22 is very tight on the bundle ofrods 24, thus forming a hard, rigid structure.

Templates 26 and 28 may be made of any suitably stiff material such, forexample, as cardboard, Masonite, sheet metal, etc. The respectiveopenings 32 and 34 provided in templates 26 and 28 may be formed thereinby hand or in any other suitable manner. The shapes of the openings 32and 34 correspond to the asymmetrical or hook shape desired for the bore12 to be produced in the propellant 10.

The several steps in the method of fabricating the tool 20 and thesubsequent use thereof, according to the present invention, comprise thefollowing:

1. Using a suitably-sized plastic bag 22 with the bottom end thereofsealed, the bag 22 is fitted inside the shaped openings 32 and 34 intemplates 26 and 28, respectively, with the templates being suitablyretained in spaced relationship along the length of the bag 22.

2. The inside of the bag 22 is filled with the rods 24.

3. The top of the bag 22 is sealed by heat sealing or other suitablemeans.

4. Using vacuum needle 30 connected to a suitable suction source, theair is exhausted from the bag 22 until the bag 22 is very tight on thebundle of rods 24.

5. The vacuum needle 30 is withdrawn from the bag 22 and the hole in bag22 made by the needle 30 is sealed by heat seal or other suitable means.

6. The templates 26 and 28 are removed from bag 22 and the containedbundle of rods 24 whereby there is produced a tool or core 20, as shownin FIG. 3, that is very rigid and hard.

7. The tool or core 20 may be used to load a rocket motor withpropellant in accordance with known solid rocket motor propellantloading procedures of the prior art as described hereinbefore.

8. Cut the bag 22 open, extract the rods 24, and peel out the plastic ofwhich the bag 22 was formed from the bore in the propellant.

As those skilled in the art will understand, while two templates 26 and28 only are shown in FIG. 2, more than two such templates may beemployed if such should be found to be necessary or desirable forfacilitating the fabrication of the tool or core 20.

Alternatively to the use of a multiplicity of rods 24 in the fabricationof the tool or core 20, a granular material such as sand indicated at 36in FIG. 6, may be employed for filling the bag 22. As shown in FIG. 6,sand 36 may be dispensed into the top opening of bag 22 from areciprocating funnel 38 to which sand is supplied from a source notshown. Even with the use of granular material 36, the tool or core 20that results upon exhaustion of the air from bag 22 is very rigid andhard. With the use of such granular material for filling bag 22,however, the employment of additional templates, as indicated by thetemplates 26a, 26b, 26c. . . 26n, to ensure the uniformity or constancyof the cross section of the tool or core 20, is desirable. If necessary,a single template having a length corresponding substantially to that ofthe bag may be employed.

It is contemplated that all of the templates 26, 28, 26a, 26b, etc. maybe removed from the bag 22 and the bundle of rods 24 or other materialcontained therein by sliding them off the ends of the stiffened andrigid bag structure resulting from the exhaustion of air therefrom.Alternatively, the templates 26, 28, 26a, 26b, etc. may be removed bycutting them away.

Thus, in accordance with the invention, there has been provided a methodof and apparatus for fabricating a tool or core to form an asymmetricalconstant cross section cavity or bore in the propellant, in a solidrocket motor. This improved method is characterized in that it enablesthe fabrication of such a tool quickly and with little special machiningcapability, as contrasted with the prior art requirement forthree-dimensional NC controlled machining capability. The presentinvention simplifies the construction of tools that are very difficultto build using the methods of the prior art. The invention also allowsgreater flexibility.

With this description of the invention in detail, those skilled in theart will appreciate that modifications may be made to the inventionwithout departing from its spirit. Therefore, it is not intended thatthe scope of the invention be limited to the specific embodimentillustrated and described. Rather, it is intended that the scope of theinvention be determined by the appended claims and their equivalents.

What is claimed is:
 1. Apparatus for fabricating a tool for forming abore of desired cross sectional form in the propellant of a rocket motorcomprising,template means providing a shaped opening corresponding tothe cross sectional form desired for the propellant bore, a multiplicityof individual solid pieces of matter, and a bag of flexible materialfitted inside the opening provided by said template means and filledwith said individual solid pieces of matter, said bag when filled beingsealed and exhausted of air until tight on the multiplicity of pieces ofmatter therein and thereby formed into a rigid structure, whereby saidtemplate means may be removed and said rigid structure may be used inthe loading of a rocket motor propellant to form a bore therein. 2.Apparatus as defined by claim 1 wherein said bag is made of plastic. 3.Apparatus as defined by claim 1 wherein the opening provided by saidtemplate means is asymmetrical in form whereby said tool may be made toform an asymmetrical constant cross section bore in the rocket motorpropellant.
 4. Apparatus as defined by claim 1 wherein said templatemeans comprises a plurality of spaced individual templates each having ashaped opening corresponding to the cross section form desired for thepropellant bore, the openings of all of said templates being positionedin alignment.
 5. Apparatus as defined by claim 4 wherein saidmultiplicity of solid pieces of matter comprise small rods each of whichhave a length corresponding substantially to that of said bag. 6.Apparatus as defined by claim 5 wherein said rods have a round crosssection.
 7. Apparatus as defined by claim 5 wherein said rods have ahexagonal cross section.
 8. Apparatus as defined by claim 4 wherein saidmultiplicity of solid pieces of matter comprise grains of sand. 9.Apparatus as defined by claim 1 wherein said multiplicity of solidpieces of matter comprise rods.
 10. Apparatus as defined by claim 1wherein said multiplicity of solid pieces of matter comprise grains ofsand.